The invention pertains to the field of bidirectional passband digital communication systems, and, more particularly to the field of improvements in head end or central office modems to remove the phase locked loops therefrom.
Digital data communication systems are well known in the art. Many treatises are available that describe them. Among these treatises are: Dixon, xe2x80x9cSpread Spectrum Systems with Commercial Applicationsxe2x80x9d, Third Edition, 1994 (Wiley and Sons, New York) ISBN 0 471 59342-7; Stallings xe2x80x9cData and Computer Communicationsxe2x80x9d, 4th Ed. 1994 (Macmillan Publishing Co., New York) ISBN0-02-415441-5; Lee and Messerschmit, xe2x80x9cDigital Communication, 2d Ed.xe2x80x9d, 1994 (Kluwer Academic Publishers, Boston), ISBN 0 7923 9391 0; Haykin, xe2x80x9cCommunication Systemsxe2x80x9d Third Edition 1994 (Wiley and Sons) ISBN 0 471 57176-8; Elliott, Handbook of Digital Signal Processing: Engineering Applications, (Academic Press, Inc. San Diego, 1987), ISBN 0-12-237075-9, all of which are hereby incorporated by reference. Generally, the problem which the invention is an attempt to solve is how to get rid of as many continuous tracking loops as possible in a bidirectional digital data communication system. The reasoning for this can be understood from the following discussion.
Digital data distributed communication systems can be baseband systems or passband systems. In baseband systems, the transmission media has the capability of transmitting digital pulses between widely separated transmitter and receiver locations. Passband systems require that the digital data be modulated onto a carrier frequency for transmission over the media.
Receivers for digital data passband systems can be either coherent or noncoherent. In coherent systems, the receiver has a local oscillator, usually taking the form of a phase locked loop (PLL) which is part of a continuous tracking loop and is maintained in constant phase lock with the phase and frequency of the carrier on which the received data is modulated. Coherent systems can make use of modulation schemes which alter either the phase, frequency or amplitude or any combination thereof of the carrier in accordance with the information content of the digital data to be transmitted. Incoherent systems do not have the local oscillator at the receiver phase locked to the carrier phase and frequency. In these systems, the designers have chosen to ignore the phase of the received signal at the expense of some degradation of the system performance and throughput.
Coherent systems can utilize binary or M-ary amplitude shift keying (ASK), phase shift keying (PSK) or frequency shift keying (FSK), as well as M-ary amplitude phase keying (APK) of which QAM (quadrature amplitude modulation) is a special case. Incoherent systems are limited to binary or M-ary ASK, FSK or differential phase shift keying (DPSK).
Coherent systems are higher performance systems because they have an additional degree of freedom for use in the modulation scheme which means more complex constellations of symbol sets can be used and more data bits can be encoded in each symbol in the constellation. This translates to greater throughput.
However, coherent systems are more complex since they require additional tracking loop circuitry at the receiver to recover the transmitted carrier and use the information so derived to steer the local oscillator so as to maintain its phase and frequency locked to the phase and frequency of the carrier. Usually the local oscillator being steered in the receiver is a PLL or has a voltage controlled oscillator negative feedback system in it (which is at the heart of almost every tracking loop). Carrier synchronization has been achieved by any one of a number of different ways in the prior art including use of PLLs where the carrier is not suppressed or Mth power tracking loops or Costas tracking Loops where the carrier is suppressed. Mth power and Costas tracking loops also contain voltage controlled oscillators as part of the tracking loop. The problem is that PLLs and negative feedback voltage controlled oscillators in tracking loops can and often do lose lock especially where there is rapid change in phase or frequency caused by conditions in the transmission media. When a PLL or other tracking loop loses lock, the system goes out of synchronization and fails to communicate dataxe2x80x94its sole purpose in life.
All digital data communication also requires clock synchronization in the receiver to the clock in the transmitter because data is sent during discrete times. These discrete times are variously called chip times, bit times or symbol times in the prior art references. The importance of synchronization of the clock in the receiver to the clock in the transmitter is that in all forms of modulation, the amplitude, phase or frequency of the carrier (or some combination of these) must be sampled during each chip time so as to determine which symbol in the alphabet or code set in use was transmitted during that chip time based upon the phase, amplitude or frequency characteristics of the carrier during that chip time.
Receiver clock synchronization can be done on either a long term basis or a short term basis. Short term clock synchronization is called, amazingly enough, asynchronous transmission, but in fact the receiver clock is periodically synchronized to the transmitter clock at the beginning of transmission of each xe2x80x9ccharacterxe2x80x9d. A character is a collection of 5 to 8 symbols which are transmitted over a very short time (usually the symbols or binary bits that only have two states). The receiver clock resynchronizes during each character at the beginning thereof and need not resynchronize until the next character starts. Asynchronous transmission is cheap and less complex since timing synchronization problems caused by transmission of long uninterrupted streams of bits is avoided by sending the bits one character at a time and requiring synchronization between the receiver clock and transmitter clock only during that character.
The problem with asynchronous transmission is the high overhead. Each character of 5 to 8 bits must include a start bit, 1 or 2 stop bits and a parity bit. The start bit is used by the receiver to resynchronize its clock. The overhead of 2-3 bits per character of 5-8 bits makes asynchronous transmission inefficient to transmit large volumes of data. Asynchronous transmission can be extended to sending several characters grouped together with a preamble which is long enough for the receiver to synchronize to transmitted before every group of characters and a tracking loop to maintain the receiver clock in synchronization with the transmitter clock during the transmission of the group of characters. The concepts of the invention are applicable to asynchronous transmission where there is a tracking loop in the remote unit receiver but no tracking loop in the central unit receiver and only a periodic or occasional phase adjustment of the master clock and master carrier phase for use by the central unit receiver.
Synchronous transmission is a more efficient way of transmitting data since blocks of symbols or bits can be transmitted without start and stop codes. Sampling by the receiver during the middle of each bit or chip time is accomplished by keeping the receiver clock in synchronization with the transmitter clock. This maintenance of clock synchronization has been done in the prior art in many different ways. For example, a separate clock line can connect the transmitter and receiver, but this is impractical in many situations. A way of avoiding this is to embed the clock information in the data signal transmitted from the transmitter and recover the clock in the receiver.
Clock recovery has been done in a number of different ways in the prior art including transmitting the clock along with the data bearing signal in multiplexed form and using appropriate filtering of the modulated waveform to extract the clock. Another method is to use a noncoherent detector to first extract the clock and then processing the noncoherent detector output to recover the carrier. Where clock recovery follows carrier recovery, the clock is recovered from demodulated baseband signals. The early-late gate symbol synchronizer has also been used in the prior art to synchronize the receiver clock to the transmitter clock. This type clock recovery takes advantage of the fact that a matched filter output of a filter matched to a rectangular clock pulse is a triangular waveform which can be sampled early before the peak and late after the peak. By changing the timing of the sampling until the early and late samples have equal amplitude, the peak of the matched filter output signal can be found, and this will have a fixed phase relationship to the clock phase. This information is then used to steer a voltage controlled oscillator in a negative feedback system. Again, complicated circuitry centered around a voltage controlled oscillator is needed to recover the clock.
A technique called remote loopback or remote clock has been used in the prior art on, for example T1 type digital data communication phone lines. This technique is similar to the aspect of the invention involving having the remote unit local clock synchronized to the central unit master clock and using that local clock at the remote unit receiver for the remote unit transmitter. It is also similar to the aspect of the invention of using the central unit master clock, after adjustment in phase to synchronize it to the phase of the received clock from the remote unit transmitter, as the clock signal from the central unit receiver.
Since PLLs and tracking loops are not always reliable, and add complication and expense to receivers, it is desirable to be able to get rid of them wherever possible. Thus, a need has arisen for a bidirectional digital communication system where continuous tracking loops in the central unit receiver (or the receiver in the unit having the transmitter which transmits with the master clock and master carrier signals) have been eliminated.
A bidirectional digital data communication system according to the teachings of the invention will have: a central unit transmitter with any encoder to receive downstream data, encode it and drive any type of digital passband modulator with the encoder receiving a master clock signal from a master clock oscillator and the modulator receiving a master carrier oscillator; a remote unit receiver which has any compatible detector which receives a local carrier reference signal which is synchronized in frequency and phase to the master carrier signal and which is generated by any form of carrier recovery circuit, with the detector driving a decoder to decode the received data and output it, with the decoder receiving a local clock signal which has been synchronized with the transmitter master clock signal by any clock recovery circuit; a remote unit transmitter having any encoder type for receiving upstream data, encoding it and driving any digital passband modulator, the encoder receiving the local clock reference generated by the remote unit receiver clock recovery circuit and the modulator receiving the local carrier reference signal generated by the remote unit carrier recovery circuit; and a central unit receiver with any compatible coherent detector to detect the signal transmitted from the remote unit transmitter, with the central unit detector using the central unit master carrier from the master carrier oscillator in the transmitter but adjusted in phase to account for propagation delay from the remote unit, and with the decoder using the master clock signal from the central unit transmitter master clock oscillator but adjusted in phase for the propagation delay from the remote unit to the central unit. Thus, the central unit has no phase locked loops or other voltage controlled oscillator circuits for clock recovery or carrier recovery.
In the preferred embodiment, the master carrier and master clock are recovered in the RUs and used to transmit data upstream along with preamble data preceding payload data. The preamble data from each RU is used by the central unit transceiver to generate an amplitude and phase correction factor for that RU. The signals from that RU are then demodulated using the CU master carrier and demultiplexed and detected using the CU master clock. Phase and amplitude errors in the detection process caused by latency and channel impairments are eliminated or reduced by using the phase and amplitude correction factors developed for this RU from its preamble data. Thus, there is no need for continuous tracking loops in the CU receiver to recover the clock and carrier used by each RU to transmit its data. This single master carrier and master clock concept and the frame synchronization provided by ranging, and the improved throughput and lower error rates provided by the equalization and power alignment processes taught herein are useful in any form of bidirectional digital data distributed communication system regardless of the form of encoding, multiplexing or modulation used. Examples of the types of multiplexing that can be used in such systems are CDMA, TDMA, inverse Fourier, DMT or any other system where orthogonal signals are used to encode each separate channel of data from a source such as sine and cosine signals etc.
In the broadest embodiment of the invention involving no continuous tracking loops in the CU receiver to recover RU clock and carrier, the type of central unit transmitter and modulation scheme is not important nor is it important whether the central unit transmits a single channel of digital data downstream or multiplexes several channels. If the central unit transmitter is a multiplexing transmitter, the type of multiplexing is not important. Likewise, the type of detector used in the remote unit receiver is not important as long as it is compatible with the modulation scheme in use and it is not critical whether the central unit transmitter transmits the master carrier or suppresses it or transmits the master clock separate or embeds it in the data so long as the master clock and carrier phase information get transmitted somehow to the RUs such as embedded in the Barker code of the preferred embodiment. Likewise, the type of carrier recovery and clock recovery circuits used in the remote unit to synchronize the local clock and local carrier oscillators to the master clock and master carrier are not critical. Also, the type of decoder used in the remote unit receiver is not critical so long as it is compatible with the type of encoder used at the central unit transmitter. For the remote unit transmitter, any type of encoder and any type of modulator may be used for the upstream data, and the type of encoding and the type of multiplexing, if any, used for the upstream direction need not be the same as the downstream direction. The clock and carrier signals used by the remote unit transmitter are the same clock and carrier signals used by the remote unit receiver.
The central unit receiver can use any type of detector that is compatible with the modulation scheme used by the remote unit transmitter. Likewise, the type of decoder used in the central unit receiver is not critical so long as it is compatible with the remote unit transmitter encoder. The structure and operation of the central unit receiver phase detection and adjustment circuit is not critical to the invention. The only requirement on this circuit is that it be able to occasionally or periodically detect any phase differential between the central unit master carrier and the carrier used to transmit by the remote unit transmitter and detect any phase difference between the central unit master clock and the clock information used to transmit the received data. These phase differences are used by the central unit receiver to occasionally or periodically adjust the phase of the master clock and master carrier to match the phases of the carrier and clock signals used by the remote unit transmitter as received at the central unit receiver.
The invention is applicable to both asynchronous and synchronous methods of transmission, although synchronous transmission is much more efficient in terms of overhead consumed per unit of payload data delivered. Use of the invention in asynchronous transmission will be useful in asynchronous systems where tracking loops are used to maintain synchronization of the remote unit receiver local clock during transmission of one or more characters in a group.
In the preferred embodiment, the transmitters of the RU use synchronous code division multiplexing (SCDMA). SCDMA is defined as transmission of frames of spread spectrum signals with data from different channels spread using orthogonal pseudorandom spreading codes, said frames being synchronously transmitted from different RUs located at diverse locations such that all frames of corresponding frame number from all RUs arrive at the CU modem with their frame boundaries exactly aligned in time with the frame boundaries of the CU frame of the same frame number. The upstream data is then demultiplexed and decoding by the inverse code transformation that was used in the RU transmitter to spread the spectrum of the data using the orthogonal, pseudorandom spreading codes.
According to the most preferred embodiment, there is provided a code division multiplexing multiple access (CDMA) scheme using orthogonal codes to encode multiple channels of digital data for simultaneous transmission over a cable television media which is also carrying frequency division multiplexed cable television programming. Further, in this most preferred embodiment, alignment of multiple subscriber remote units at diverse locations on the cable television media to the same frame alignment reference is used to substantially reduce crosstalk between adjacent codes and allow multiple users to simultaneously share the same cable TV media for auxiliary services other than cable TV programming delivery. The ranging process described herein is useful for any digital communication system which delivers data from physically distributed transmitters to a central location in frames, but in the context of a CDMA system on a cable TV plant, it provides for synchronous CDMA which greatly increases system payload capacity. The use of synchronous CDMA coupled with frequency division multiplexing of upstream and downstream data on different frequencies than the cable TV programming provides a system whereby the entire bandwidth devoted to the digital auxiliary services may be simultaneously shared by multiple users who share a plurality of channels. Any of the known ways of achieving frame alignment may be used to achieve synchronous code division multiple access data transmission. In the preferred embodiment, frame alignment is achieved with the bulk of the processing done by the RUs with the CU only acting in a passive role as a sensor for deciding if a Barker code is in the gap, if there is more than one Barker code in the gap, asking for authentication and providing feedback for all of the above and for fine tuning processing to exactly center each RU""s Barker code in the gap. This ranging process is done by alignment of ranging signals transmitted by remote units to guardbands or gaps between frames.
One inventive concept disclosed herein is to achieve high noise immunity by spreading the energy of the transmitted data out over time during transmission, and then compressing the energy again at the receiver to recover the data. Spreading the energy of the transmitted data out over time reduces susceptibility to burst errors and impulse noise. In addition to this spreading concept, the spectral efficiency of the system is enhanced by transmitting multiple separate channels of data over the same media without interference by using separate orthogonal codes to encode the data of each channel so that no interference results when all channels are simultaneously transmitted so long as proper frame alignment is maintained. In this way, the spectral efficiency, i.e., a measure of the amount of data that can be sent from one place to another over a given bandwidth, is enhanced without degradation of the data by crosstalk interference. The orthogonality of the codes used for each data stream, i.e., each channel or conversation, minimizes crosstalk between channels where the system is properly aligned, i.e., synchronized.
Using cyclic, orthogonal codes for SCDMA further enhances noise abatement by providing the ability to perform equalization using a subset of these codes. Equalization, as that term is used herein, refers to the process of determining the amount of crosstalk between adjacent codes resulting from minor errors of frame timing alignment and then generating phase and amplitude correction factors which can be used to negate the crosstalk. In the preferred embodiment, the orthogonal codes are cyclic codes.
In some species within the genus of the invention, code diversity is used to achieve further noise immunity. It has been found that some orthogonal codes are less immune to narrow band interference and other sources of noise than others. To avoid using such codes to spread the data from the same channel or timeslot all the time, code hopping is used in these preferred species of the inventive genus. Code diversity is achieved in several different ways, but, in the preferred embodiment, each transmitter uses a code shuffler circuit and each receiver uses a code deshuffler circuit. All shuffler and deshuffler circuits receive the same seed and generate the same sequence of pseudorandom numbers therefrom. These pseudorandom numbers are used to generate read pointers to a framer memory and write pointers to a buffer memory. The framer memory is where the information vectors or symbols are stored, and the read pointers generated by the shuffler circuits are used to read the timeslot data, i.e., symbol/information vector elements out in pseudorandom fashion and store them in a buffer in accordance with the write pointers generated by the code hopping shuffler circuit. The information vector elements thus stored in the buffer are used to do the matrix multiplication required by the code division multiplexing scheme. Alternatively, the symbol elements may be read out sequentially from the framer memory and stored pseudorandomly in the buffer.
The effect of this synchronous CDMA scheme is to xe2x80x9cwhitenxe2x80x9d the noise sources such that no matter how complex the noise signals, the noise can be effectively managed using conventional error detection and correction bits in a forward error correction scheme. The digital data providing the interactive or bidirectional data communication is sent using a CDMA scheme, but for purposes of synchronization, the CDMA scheme is mixed with a TDMA scheme. More precisely, a guardband or gap which is free of data is added between frames of the CDMA signal. Digital data is transmitted in frames, each frame comprising 3 data symbols and a guardband. The guardband is used for non-data usage such as ranging, alignment and equalization.
The synchronous CDMA modulation scheme disclosed herein may be used with any shared transmission media and with any apparatus or method that can get all remote units synchronized to the frame timing of the central unit including the ranging/alignment scheme disclosed herein. Other possible methods of synchronizing to the same frame timing are for all remote units and the central unit to receive the same timing reference signals from some source such as internal atomic clocks or from an external source such as a Global Positioning System satellite from which all remote units and the central unit are effectively equidistant.
Likewise, the ranging/alignment scheme disclosed herein is useful for any other modulation scheme which transmits digital data in frames, requires frame synchronization and can insert a guardband between the frames.
Some species within the inventive genus use M-ary modulation code division multiplexing. Each remote unit receives a time division multiplexed stream of digital data. Each timeslot contains 9 bits of data. Each 9 bits is stored in a framer memory, and is divided into three tribits, each having 3 bits during readout of the memory. Each of the three symbols transmitted each frame is comprised of 144 of these tribits, one for each timeslot or channel. These tribits are encoded with a 4th bit prior to spreading by the code division multiplexing operation. The 4th bit is added by a Trellis forward error correction encoder to each tribit based upon the three bits of the tribit and based upon the previous state for this timeslot""s data during the last frame. This 4th bit adds sufficient redundancy to enable a Viterbi Decoder in the central unit receiver to make a more error free determination of what data was actually sent in the presence of noise without requesting retransmission. The 4th bit also maps each tribit to a 16 point QAM (quadrature amplitude modulation) constellation by using the first two bits to represent the inphase or I axis amplitude and the last two bits to represent the quadrature or Q axis amplitude. Thus, M-ary modulation is used to achieve greater spectral efficiency.
With the system described herein, full 10 megabit/second traffic volume per each 6 MHz channel can be achieved in both the upstream and downstream direction over HFC. Unlike conventional CDMA, SCDMA transmission from transmitters like those disclosed herein stays within 6 MHz bands that do not interfere with or effect other adjacent channels. SCDMA has a number of other advantages over pure FDMA and TDMA systems in terms of capacity, scalability and bandwidth allocation. Standard IS-95 asynchronous Code Division Multiple Access spread spectrum systems are hindered by the capacity constraints of the 5-40 MHz upstream channel and the presence of a large amount of noise, and they often require 30 MHz wide channels which creates channel interference problems with neighboring services in the HFC spectrum. The biggest problem with asynchronous CDMA systems is self-generated noise because the RUs are not aligned with each other thereby losing orthogonality and creating a high degree of mutual interference. The higher self-generated noise raises the noise floor and reduces the capacity. SCDMA system insure that the RUs are in frame synchronization with each other and using orthogonal codes to minimize mutual interference as data is sent upstream. Preferably, SCDMA transmitters are also used to send data downstream. With the system described herein, multiple streams of digital data, each having a 64 kbps throughput can be simultaneously sent over a 6 MHz channel with a total 10 Mbps throughput. Each data stream is Trellis encoded, interleaved and spread over the entire 6 MHz using its own individual spreading code. Use of forward error correction and interleaving increases noise immunity to impulse noise, narrowband interference and Gaussian noise. The Trellis coding adds 4.8 dB coding gain, and interleaving enables withstanding long duration impulse noise of up to 100 microseconds without incurring errors. Use of spread spectrum technology adds another 22 dB processing gain. The combination of techniques yield a total 27 dB interference rejection allowing the system to operate in negative Carrier to Noise Plus Interference Ratio. The SCDMA transmitters are combined with TDMA payload data input streams which makes the system extremely scalable.
The high capacity of the SCDMA system disclosed herein is made possible by orthogonality which is made possible by the orthogonality of the spreading codes which is a result of the ranging process and the equalization process. The ranging process assures frame synchronization such that all codes arrive from distributed RUs arrive at the CU at the same time. The ranging process is carried out periodically to account for cable expansion/contraction with changing temperature, but the process is transparent to payload traffic in that it does not slow it down, stop it or cause errors. Re-ranging occurs upons certain error conditions and upon disconnect from the network and each powerup.
Equalization is achieved by measuring the channel response from each user to the CU and adjusting a precoder at the RU transmitter to xe2x80x9cinvert the channelxe2x80x9d, i.e., predistort the transmitted signal such that it arrives undistorted at the CU. Power alignment by each RU such that each RU transmission reaches the CU at approximately the same power level also helps to minimize mutual interference.
Dynamic bandwidth allocation allows as many 64 kbps streams or channels as necessary to be allocated to a particular service so that high demand applications such as video teleconferencing or high speed internet access can be supported simultaneously with low demand applications like telephony over the same HFC link. Bandwidth allocation is managed at the CU through an activity status table in each RU and the CU that indicates the status of each timeslot and code assignments. The CU updates the RU tables by downstream messages. Bandwidth can be guaranteed upon request while other services with more bursty traffic may contend for the remainder of the total 10 Mbps payload.
The advantages over TDMA systems include less need for fast acquisition and correspondingly lower sensitivity to narrowband interference. Further, below a certain SNR, TDMA systems may fail altogether. Contention for certain channels and contention affecting adjacent can cause amplifier overload in TDMA systems and can cause severe throughput and performance problems. FDMA systems where each user gets a narrow upstream frequency slice is very susceptible to narrowband noise which can wipe out an entire channel. FDMA systems often try to avoid this problem with frequency reallocation. This complicates and raises the cost of the system by requiring more intelligence. Throughput is also adversely affected as nothing is sent while frequencies are reallocated. Guardbands between channels waste bandwidth, and frequency misalignment degrades FDMA systems.
Any method or apparatus that uses these inventive concepts is within the teachings of the invention and is deemed to be equivalent to the apparatus and methods described herein.